Nectar robbing is a foraging behavior used by some organisms that feed on floral nectar, carried out by feeding from holes bitten in flowers, rather than by entering through the flowers' natural openings. Nectar robbers usually feed in this way, avoiding contact with the floral reproductive structures, and therefore do not facilitate plant reproduction via pollination. Because many species that act as pollinators also act as nectar robbers, nectar robbing is considered to be a form of exploitation of plant-pollinator mutualism. While there is variation in the dependency on nectar for robber species, most species rob facultatively (that is, to supplement their diets, rather than as an absolute necessity). The terms nectar theft and floral larceny have also been used in literature.

Nectar robbers vary greatly in species diversity and include species of carpenter bees, bumblebees, stingless Trigona bees, solitary bees, wasps, ants, hummingbirds, and some passerine birds, including flowerpiercers. Nectar-robbing mammals include the fruit bat and Swinhoe's striped squirrel, which rob nectar from the ginger plant.

Records of nectar robbing in nature date back at least to 1793, when German naturalist Christian Konrad Sprengel observed bumblebees perforating flowers. This was recorded in his book, The Secret of Nature in the Form and Fertilization of Flowers Discovered, which was written in Berlin. Charles Darwin observed bumblebees stealing nectar from flowers in 1859. These observations were published in his book The Origin of Species.

Nectar robbing is specifically the behavior of consuming nectar from a perforation (robbing hole) in the floral tissue rather than from the floral opening. There are two main types of nectar robbing: primary robbing, which requires that the nectar forager perforate the floral tissues itself, and secondary robbing, which is foraging from a robbing hole created by a primary robber.

The former is performed most often on flowers whose nectar is concealed or hard to reach. For instance, long flowers with tubular corollas are prone to robbing. Secondary robbers often do not have suitable mouth parts to be able to create penetrations into the flowers themselves, nor to reach the nectar without robbing it. Thus they take advantage of the perforations already made by other organisms to be able to steal the nectar. For example, short-tongued bees such as the early bumblebee (Bombus pratorum) are unable to reach the nectar located at the base of long flowers such as comfreys. In order to access the nectar, the bee will enter the flower through a hole bitten at the base, stealing the nectar without aiding in pollination. Birds are mostly primary robbers and typically use their beaks to penetrate the corolla tissue of flower petals. The upper mandible is used to hold the flower while the lower mandible creates the hole and extracts the nectar. While this is the most common method employed by bird species, some steal nectar in a more aggressive manner. For example, bullfinches reach the nectar by completely tearing the corolla off from the calyx. Mammal robbers such as the striped squirrel chew holes at the base of the flower and then consume the nectar.

The term "floral larceny" has been proposed to include the entire suite of foraging behaviors for floral rewards that can potentially disrupt pollination. They include "nectar theft" (floral visits that remove nectar from the floral opening without pollinating the flower), and "base working" (removing nectar from in between petals, which generally bypasses floral reproductive structures). Individual organisms may exhibit mixed behaviors, combining legitimate pollination and nectar robbing, or primary and secondary robbing. Nectar robbing rates can also greatly vary temporally and spatially. The abundance of nectar robbing can fluctuate based on the season or even within a season. This inconsistency displayed in nectar robbing makes it difficult to label certain species as "thieves" and complicates research on the ecological phenomenon of nectar robbing.

Pollination systems are mostly mutualistic, meaning that the plant benefits from the pollinator's transport of male gametes and the pollinator benefits from a reward, such as pollen or nectar. As nectar robbers receive the rewards without direct contact with the reproductive parts of the flower, their behaviour is easily assumed to be cheating. However, the effect of robbery on the plant is sometimes neutral or even positive. For example, the proboscis of Eurybia elvina does not come in contact with the reproductive parts of the flower in Calathea ovandensis, but this does not lead to significant reduction in fruit-set of the plant. In another example, when 80 percent of the flowers in a study site were robbed and the robbers did not pollinate, neither the seed nor fruit set were negatively affected.

The effect of floral-nectar robbing on plant fitness depends on several issues. Firstly, nectar robbers, such as carpenter bees, bumble bees and some birds, can pollinate flowers. Pollination may take place when the body of the robber contacts the reproductive parts of the plant while it robs, or during pollen collection which some bees practice in concert with nectar robbing. The impact of Trigona bees (e.g. Trigona ferricauda) on a plant is almost always negative, probably because their aggressive territorial behaviour effectively evicts legitimate pollinators. Nectar robbers may change the behaviour of legitimate pollinators in other ways, such as by reducing the amount of nectar available. This may force pollinators to visit more flowers in their nectar foraging. The increased number of flowers visited and longer flight distances increase pollen flow and outcrossing, which is beneficial for the plant because it lessens inbreeding depression. This requires a robber's not completely consuming all of a flower's nectar. When a robber consumes all of a flower's nectar, legitimate pollinators may avoid the flower, resulting in a negative effect on plant fitness.